In the field of optical telecommunications components, monolithically integrated optical circuits are widely used. Such optical circuits commonly comprise a plurality of optical waveguides (e.g. ridge waveguides) connected by means of optical splitters, at which light is split or recombined to or from different branch waveguides. Examples of such optical circuits are Mach-Zehnder interferometer (MZI) devices, such as Mach-Zehnder modulators (MZMs). MZMs are used to modulate an optical signal with an electrical data signal by splitting the optical signal into two components, phase modulating one component relative to the other, and recombining the components. With appropriate choice of the phase modulation, constructive and destructive optical interference between the components will take place on recombination, resulting in an amplitude modulated combined signal. An example of a monolithically integrated optical circuit in which two MZMs are monolithically integrated by means of an optical splitter is illustrated in the optical differential quaternary phase shift key (ODQPSK) encoder disclosed in U.S. Pat. No. 7,116,460. Further examples of MZI devices include Mach-Zehnder switches and Mach-Zehnder pulse carvers.
Multimode interference (MMI) couplers are commonly used as the optical splitters and recombiners in MZMs. In such devices light from a substantially single mode waveguide passes through a port into an MMI region comprising a multimode waveguide that describes an optical interference region, and out through one or more ports at the opposite end of the MMI region. Passage of the light into the MMI region excites optical modes of the MMI region, which optically interfere, periodically re-imaging along the length of the MMI region, and forming a terminal re-imaging through the ports at the end correspond with the terminal re-imaging pattern.
In a typical simple MMI design the MMI region is rectangular in plan, and is many times longer than it is wide. The intensity profile of the re-imaging pattern is governed by several factors such as the length and width of the MMI region. The re-imaging pattern can be further governed by a non-rectangular shape of the MMI region, for example having a constricted (“butterfly” type) or enlarged (“barrelled” type) waist half-way along the length of the region, as disclosed in “New 1×2 Multi-Mode Interference Couplers with Free Selection of Power Splitting Ratios” by Besse et al. in a paper to the 1994 European Conference on Optical Communication.
The optical splitting performance of such MMI couplers devices is limited by the fabrication accuracy of the MMI region. Further the optical splitting is susceptible to thermally induced and stress induced performance variations. Accordingly, the fabrication tolerances of the MMI impact significantly upon the manufacturing yield of the optical circuits in which they are integrated. Further, the optical performance of MMIs and integrated optical circuits into which MMIs are incorporated can drift during operational life, due to variations in ambient conditions and the effects of ageing. In an MZM, deterioration in optical performance can reduce the extinction ratio of the amplitude modulated optical output.
As is disclosed in JP2001-183710 by Yazaki et al. and GB2438222 the re-imaging pattern in an MMI region can be controlled by the use of tuning electrodes provided on the surface of the region. The tuning electrodes are positioned to lie over one or more of the main re-imaging nodes of the region, and in cooperation with a back electrode, can be used to modify the refractive index of the underlying waveguide material by means of an electro-optic effect.
Changing the refractive index changes the optical path length of the corresponding part of the multimode waveguide, inducing a phase shift in the light passing beneath the electrode. By inducing such phase changes in the light in the MMI region the subsequent re-imaging patterns are modified, in particular changing the terminal re-imaging pattern and thereby modifying the output power intensities that exit the MMI region through the different ports. In this way the optical split ratio of the transmitted light can be controlled.
Disadvantageously, the current that passes between the electrodes on top of the MMI region and the back electrode spreads, broadening the region in which the refractive index occurs to be wider than the extent of the tuning electrodes. This current spreading reduces the current density closest to the re-imaging node, compensating for which requires an increased total current injection, leading to a corresponding increase in the thermal dissipation of the device. Further, the spreading of the current reduces optical performance of the tunable optical splitter by causing refractive index changes in undesirable parts of the MMI region.
Thus there remains a need for an improved design of tunable optical splitter, which seeks to address at least some of the disadvantages of the prior art designs.
The paper “Versatile multimode interference photonic switches with partial index-modulation regions” by Yagi et al., published in Electronics Letters, Vol. 36, No. 6, pp 533-534, describes mathematical modelling of the tuning performance of tunable optical splitters. Accordingly, the ideal operating conditions can be determined for controlling a tunable optical splitter in accordance with the modelling results to give the desired splitting performance.
Alternatively, the optical performance of a tunable MMI coupler that is used in a tunable optical splitter can be determined by monitoring the light emitted from the output waveguides at the output facet, and the control conditions for the coupler can be set accordingly. However, such a method requires the use of external power monitoring components, which are difficult to align with the output light, increasing complexity and associated expense of this step within an industrial manufacturing process.
Further, the present inventors have appreciated that the performance of a tunable optical splitter will be susceptible to the effects of manufacturing variations in the fabrication of the MMI region and deposition of the electrodes. Further, the performance will vary with ambient temperature, mechanical stress and the effects of ageing over life, such as creep in the electrode metallization. Accordingly a tunable MMI coupler will not necessarily have an optical performance response in accordance with the modelling or remain constant in its response over life.
Thus there remains a need for an apparatus adapted to enable control of an MMI device having a tunable split ratio and a corresponding control scheme, which seeks to address at least some of the disadvantages of the prior art.
Optical splitters having MMIs with fixed split ratios are used as the optical splitters and recombiners in conventional optical waveguide Mach-Zehnder interferometers, such as Mach-Zehnder modulators (MZMs). Different applications require MZIs having different optical properties and it is necessary to choose the correct hardware from a variety of optoelectronic modules with different optical properties, necessitating the manufacture and storage of an inventory of different parts. The optical performance of such MZIs is subject to variations in fabrication, ambient conditions and the effects of ageing. Further, if the requirements of the application should change, it would be necessary to change the hardware in order to provide a module with different optical properties.
In long distance optical telecommunications applications, the refractive index of optical fibres commonly varies as a function of the wavelength of light transmitted. High data rate optical signals have a substantial bandwidth, causing optical signals transmitted along long optical fibres to suffer from deterioration of the signal quality due to chromatic dispersion. Although some telecommunications applications require the use of zero chirp, in other telecommunications applications it also is common to mitigate such deterioration in the signal quality by applying chirp to the optical output.
In MZMs, the extinction ratio and chirp performance of the MZM are a function of the optical split ratios of the MMI regions used in the optical splitters and recombiners of the MZM. Imbalanced (i.e. unequal) split ratios are used to provide non-zero chirp in the optical output of the MZM.
Thus there remains a need for an MZI having a controllable split ratio of the optical splitter or optical recombiner, and for an MZM having a controllable chirp, in which the level of chirp is controlled by means of controlling the split ratio of the optical splitter and/or recombiner.
U.S. Pat. No. 6,571,038 and the paper, “Multimode Interference Couplers with Tunable Power Splitting Ratios” by Joyner et al published in Journal of Lightwave Technology, Vol. 19, No. 5, pp-700-707, 2001, describe a multimode interference coupler having a tunable power splitting ratio and a method of tuning the splitting ratio. The tuning of the power splitting ratio is achieved by varying an effective refractive index around a portion in a multimode interference section.